"It is not only fatigue. One of the small reasons was fatigue, one of the main reasons was I have never been a fantastic player on the indoor surface. Also I have been unlucky," the world No. 1 said at his pre-tournament press conference at London's O2 Arena.

Only two of Nadal's 60 career titles have come indoors, and with clay being his favoured surface thanks to winning a record eight French Open crowns, the 27-year-old feels the tournament organisers should consider playing the event on the red dirt, although he admits it is something that is unlikely to happen while he is still competing.

"The Tour Finals have been indoors from 2005 until now, so I am a bit unlucky with this. For me it is more fair to have it outdoors on different surfaces," he said. "In this tournament we qualify by playing on all surfaces, but the Tour Finals are always on hard courts.

"I know it won't happen in my generation. It's not for me. I say it for the next generation and because it would be interesting for the fans."

Nadal was quick to point out that playing at the O2 is one of the best venues on the ATP calendar, but then emphasised his belief the event should move to clay.

"This is a great place to play, I have never been to a tournament with a better atmosphere, but that doesn't mean the ATP can't be a little bit more fair with the players," Nadal said.

"We could change every year to play it on the surfaces we qualify for the Finals. That means, for example, a good clay-court player would have the chance to play on his best surface."